Pressure gradient noise canceling microphone



Jan. 19, 1960 w. A. BEAVERSON 2,921,993

PRESSURE GRADIENT NOISE CANCELING MICROPHONE Filed Oct. 4, 1955 f NIGER United States Patent PRESSURE GRADIENT NOISE CANCELING NIICROPHONE Wayne A. Beaverson, Niles, Mich., assignor to Electro- Voice, Incorporated, Buchanan, Mich.

Application October 4, 1955, Serial No. 538,395

23 Claims. (Cl. 179-121) This invention relates to directional noise canceling microphones, and more particularly to such microphones having only one diaphragm.

A number of directional microphone systems utilizing gradient principles have been described in various media in recent years. In general, increased directivity has been achieved only by increasing the order of the gradient involved. This invention relates to a microphone which improves the directivity by utilizing an interference principle rather than a gradient principle. This application is a continuation in part of the inventors application entitled Directional Noise Cancelling Microphone, filed April 22, 1955, Serial No. 503,159, now abandoned. The idea may be applied to both pressure and gradient microphone systems to improve the directivity index. The invention consists of using two openings (or series of openings) on one side of a microphone diaphragm on a line which is spaced so that the distance between the two openings (or effective distance between two series of openings) is approximately equal to the half wave length of a frequency (or band of frequencies) which is to be discriminated against.

One application of this invention is to control or minimize feedback in public announcing systems. In general, a band of frequencies slightly greater than two octaves is troublesome in this sort of application. In order to meet public announcing system requirements, means are provided to extend the range of frequencies which will be discriminated against without creating a microphone which is too directional at the higher frequencies. This desirable feature is achieved by providing a microphone of the type described above with a plurality of sound entrances of the proper sizes for each of the spaced openings, each of which entrances will be chosen to optimize the discrimination against a particular frequency band.

It is accordingly an object of this invention to provide a new and improved directional noise canceling microphone.

It is another object of this invention to provide an improved microphone effective to discriminate against unwanted sounds over a greater range of frequencies.

Still another object of this invention is to provide an improved microphone which will reproduce sound in conjunction with other equipment at a higher level in a public address system before audible oscillatory feedback occurs.

Other objects and advantages of the invention will be apparent during the course of the following description when read in connection with the accompanying drawings, wherein:

Figure l is a cross-sectional view of a first embodiment of a microphone having multiple frequency discrimination;

Figure 2 is a cross-sectional view of a second embodiment of a microphone having multiple frequency discrimination;

- openings 16.

ice

Figure 3 is a cross-sectional 'iew'of a third embodiment of a microphone having multiple frequency discn'mination; and

Figures 4 and 5 are top plan views of Figures 1 and 3 showing two possible arrangements of the sound entrances.

Referring now more particularly to Figure 1 for a first embodiment of this invention, there is shown a directional pressure responsive microphone having a housing wall 10. Housing wall 10 is covered by a front housing closure 11 and a rear housing closure 12. The front housing closure 11 has a concave or cone-shaped depression '13 with the vertex of the cone at the center of the front housing closure. The rear housing closure 12 has a protrusion 14 which is located with its axis at the center of the rear housing closure.

The front housing closure 11 has two groups of openings 15 and 16 which act as sound entrances through this front housing closure. The groups of openings 15 and 16 are composed of eight pairs of openings 17 through 24, one opening of the respective pairs being located in the group of openings 15 and the other opening of the respective pairs being located in the group of The two groups of sound openings 15 and 16 are positioned diametrically opposite each other. The effective spacing between the two groups of openings is approximately inversely proportional to frequency, that is, the frequency discriminated by each pair decreases going from the inner pair of sound entrances 17 toward the outer pair of sound entrances 24. By a proper choice of the diameter, spacing and length of each pair of sound entrances, the effective spacing between the two groups of openings, which is a function of frequency, can be made equal to a half wave length over a considerable range of frequencies.

The length of the sound entrances 15 and 16 is varied by making the front housing closure progressively thicker going from the edge of the cone-shaped depression to its periphery by slanting one surface 25 of the front closure housing 11 to form an acute angle with outer surface 26 of the front closure 11. A plate 27, having a central opening 28, is mounted within the housing on housing wall 10. It is substantially parallel to the rear housing closure 12. Positioned within the central opening 28 is a diaphragm 29, the surface of which is substantially parallel to the cone-shaped depression 13 in the front housing closure 11. A transmitter 30 has one end connected to the center of the diaphragm 29. The other end of transmitter 30 is attached to a piezoelectric or transducer element 31 which is mounted in the protrusion 14. An appropriate pickup wire is brought out of the housing from the piezoelectric element 37 in any well known manner.

For a second embodiment of a microphone employing the present invention, reference is had to Figure 2 showing a pressure gradient type of microphone. A housing wall 40 having a front housing closure 41 and a rear housing closure 42 forms the microphone housing. The front housing closure 41 has a cone-shaped depression 43 with the vertex of the cone on a vertical axis through the center of the front housing closure. The rear housing closure 42 has a protrusion 44 which is located with its axis at the center of the rear housing closure. The front housing closure 41 has two groups of openings 45 similar to groups 15 and 16 of Figure l. The relationship of the pairs of openings of the sound entrances 45 and their relative arrangement is the same as set out in the description of the front closure openings 15 and 16 of Figure 1. The rear housing closure 42 has two groups of openings 46 which act as sound entrances through the rear housing closure. The openings 46 are positioned in alignment with the openings of front closure 41. The

Patented Jan. 19, 1960 as those of groups 15 and 16 in Figure 1 since the innersurface .47 of the front housing closure 41 isd-isposed at an angle-to the face of they front closure 41 causing it to become progressively thicker from the edge of the cone-shaped depression 43 outward toward its periphery where it meets the housingwall 40.

1 In the same manner,*the.inner surface 43 of the rear housing closure 42 becomes progressively thicker from theedge of the cylindrical protrusion 44 to its periphery where it meets the housing wall 40. An acoustical phase shifting impedance 49 is attached to bosses i) and 51 projecting from the rear housing closure, and is positioned to cover the two groups of sound openings 46 in the rear housing closure. These phase shifting impedances provide for the phase shift necessary to obtain optimum discrimination against the selected frequency. A plate 52, having a central opening 53, is mounted within the housing on housing wall 40. Positioned within the central opening 53 is a diaphragm 54, the surface of which is substantially parallel to the cone-shaped depression 43 in the front housing closure 41. A transmitter 55 has one end connected to the vertex of the diaphragm 54. The other end of the transmitter 55 is attached to a piezoelectric or transducer element 56 which is mounted in the protrusion 44. An appropriate pickup wire is brought out of the housing from thetransducer element 56 in any well known manner. The effective spacing between each pair of sound openings of groups or entrances 45 and 46 is selected in the same manner as that for the groups 15 and 16 in Figure 1.

In operation, whenever sound strikes the microphone along an axis passing through openings 45 and 46 (and a symmetrical pair), the operation will be similar to that of the well known cardioid microphone and will not be described here. When sounds strike the microphone from a direction perpendicular to the case described above, the operation changes. In this case, the two gradient acoustical resultants (from openings 45 and 46 and .a symmetrical pair) add together vectorially within the microphone to give a resultant which is different than that of ordinary cardioid microphones. At the frequency for which the effective D is equal to a half wave length, the output will be zero. By proper choice of acoustical constants, the effective spacing of D may be made to varyin a manner which allows it to equal a half wave length over a reasonable band of frequencies. As the direction of sound origin deviates from a line which joins the symmetrical pairs of openings, the response approaches that of the cardioid. It is easily seen that such a microphone will have a directivity which exceeds that of the ordinary cardioid microphone at and above a certain band of frequencies.

a For most effective noise discrimination against a particular frequency, the microphone dimensions for a particular pair of sound openings are therefore such that the value of the effective spacing D for that pair of sound openings is approximately equal to one-half the wave length of that frequency. The distance L between the front and the rear sound entrances is made small in comparison to the wave length of the highest frequency to be transmitted. Since the effective spacing D for each pair of sound entrances may be chosen to discriminate against a particular frequency band, this multi-frequency discrimation microphone will be effective to discriminate against unwanted frequencies over a greater band of ire since this length is constant in this embodiment.

front housing closure 61 has a concave or cone-shaped depression 63 with the vertex of the cone on a vertical axis through the center of the front housing closure. The rear housing closure 62 has a protrusion 64 which is located with its axis at the center of the rear housing closure. The front housing closure 61 has two groups of openings 65 which act as sound entrances through this front housing closure. Sound entrances 65 are arranged in the same manner as the groups of sound openings 15 and 16 of Figure 1. They differ from groups 15 and 16 in that the effective spacing between-pairs of sound entrances is determined without considering the length of the holes, For example, impedances which are essentially stiifnesses at low frequencies may be placed over the sound entrances near the center to get a different effective D. The above would increase the useful frequency band by decreasing the lower extremity of frequencies. The rear housing closure 62 has two groups of sound openings 66 which act as sound entrances through the rear housing closure. The relationship between sound entrance openings 66 and 65 is the same as that for the groups of sound en-.

trance openings 46 and 45 of Figure 2.

An acoustical phase shifting impedance 67 is mounted on projections 68 and 69 and positioned to cover the openings 66. These phase shifting impedances provide for the phase shift necessary to obtain optimum discrimination against the selected frequencies. A plate 70 having a central opening 71 is mounted within the housing on housing Wall 60. Positioned within the central opening 71 is a diaphragm 72, the surface of which is substantially parallel to the cone-shaped depression 63 in the front housing closure 61. A transmitter 73 has one end connected to the center of the diaphragm 72. The other'end of the transmitter 73 is attached to a transducer element which is mounted in the depression in the same manner as described and shown in connection with the embodiment of Figure 2. The transducer element'may be any one of the well known types. The operation of the embodiment of Figure 3 is similar to the operation described above for the embodiment of Figure 2,

' While this invention has been shown and described in since it is apparent that the principles herein disclosed are susceptible of numerous other applications, and modifications may be made in the circuit arrangement and in the instrumentalities employed withoutdeparting from the spirit and scope of this invention as set forth in the apcylindrical protrusion, said rear housing closure becoming progressively thicker from the edge of the cylindrical protrusion to its periphery, a plate having an opening con-t nected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate having a second depression of substantially similar shape as and with its surface substantially parallel to the surface of said first depression, a transducer element mounted in said protrusion and connected to said diaphragm, said front closure having a first plurality of passages, said front closure having a second plurality of pets sages located substantially diametrically opposite said first plurality of passages, each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave 2i length of a particular chosen frequency, said rear closure having a first plurality of passages in substantial alignment with said first plurality of passages intsaid front closure, a second plurality of passages substantially diametrically opposite said first plurality of passages in substantial alignment with said second plurality of passages in said front closure, the relationship as to eifective spacing and impedance between passages in said rear closure being the same as that between corresponding substantially aligned passages in said front closure, a first phase shifting element covering said first plurality of passages in said rear closure, and a second phase shifting element covering said second plurality of passages in said rear closure.

2. A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a first depression in said front housing closure, said front housing closure becoming progressively thicker from the edge of said depression to its periphery, a rear housing closure attached to the other end of said wall having a protrusion, said rear housing closure becoming progressively thicker from the edge of the protrusion to its periphery, a plate having an opening connected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate having a surface substantially parallel to the surface of said first depression, a piezoelectric element mounted in said protrusion and connected to said diaphragm, said front closure having a first plurality of passages, said front closure having a second plurality of passages located substantially diametrically opposite said first plurality of passages, each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a first plurality of passages equal in number and spacing from the periphery as said first plurality of passages in said front closure, a second plurality ofpassages substantially diametrically opposite said first plurality of passages equal in number and spacing from the periphery as said second plurality of passages in said front closure, the relationship as to effective spacing and impedance between passages in said rearclosure being the same as that between corresponding passages in said front closure, and phase shifting elements covering said plurality of passages in said rear closure.

3. A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a first depression in said front housing closure, said front housing closure becoming progressively thicker from the edge of said depression to its periphery, a rear housing closure attached to the other end of said wall having a protrusion, said rear housing closure becoming progressively thicker from the edge of the cylindrical protrusion to its periphery, a plate having an opening connected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate having a surface substantially parallel to the surface of said first depression, a transducer element mounted in said protrusion and connected to said diaphragm, said front closure having a first plurality of passages, said front closure having a second plurality of passages located substantially diametrically opposite said first plurality of passages, each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to onehalf of the wave length of a particular chosen frequency, said rear closure having a first plurality of passages equal in number and spacing from the periphery as said first plurality of passages in said front closure, a second plurality of passages substantially diametrically opposite said first plurality of passages equal in number and spacing from-the periphery as said plurality of passages in said front closure, the relationship as to effective spacing and impedance between passages in said rear closure being the same as that between corresponding passages in said front closure.

4. A microphone comprisinga housing wall, a front housing closure attached to one end of said wall having a convex inner surface portion, said front housing closure becoming progressively thicker from the edge of said convex surface to its periphery, a rear housing closure attached to the other end of said wall, said rear housing closure becoming progressively thicker outwardly toward its periphery, a plate having an opening connected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate having a surface substantially parallel to the surface of said first depression, a transducer element mounted in said rear housing closure and connected to said diaphragm, said front closure having a plurality of passages extending therethrough, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a plurality of passages equal in number and spacing apart as said plurality of passages in said front closure, the relationship as to efiective spacing and impedance between passages in said rear closure being the same as that between corresponding passages in said front closure.

5. A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a convex inner surface portion, said front housing closure becoming progressively thicker from the edge of said convex surface to its periphery, a rear housing closure attached to the other end of said wall, said rear housing closure attached to the other end of said wall, said rear housing closure becoming progressively thicker outwardly toward its periphery, means to divide the housing into two chambers including a diaphragm mounted in said opening of said plate having its sinface substantially parallel to the surface of said convex inner surface, a transducer element mounted in said rear housing closure and connected to said diaphragm, said front closure having a plurality of passages extending therethrough, each of said passages being of such diameter, length and distance apart as to have an effective spacing equal to onehalf of the wave length of a particular chosen frequency, said rear closure having a plurality of passages associated with said plurality of passages in said front closure, the relationship as to effective spacing and impedance between passages in said rear closure being the same as that between corresponding associated passages in said front closure, and a phase shifting element covering said plurality of passages in said rear closure.

6. A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a convex inner surface portion, said front housing closure becoming progressively thicker from the edge of said convex surface to its periphery, a rear housing closure attached to the other end of said wall, said rear housing closure becoming progressively thicker outwardly toward its periphery, means to divide the housing into two chambers including a diaphragm mounted in said opening of said plate having its surface substantially parallel to the surface of said convex inner surface, a piezoelectric element mounted in said rear housing closure and connected to said diaphragm, said front closure having a first plurality of passages, said front closure having a second plurality of passages, each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a first plurality of passages associated with said first plurality of passages in said front closure, a second plurality of passages associated with said second plurality of 'passages in said front closure, therelationship as to effective spacingand' impedance. between passages'in said rear closure being the same as that between corresponding associated passages in said front closure,

A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a" first concave depression in said front housing closure, said front housing closure becoming progressivelythicker from the edge of said cone-shaped depression to its periphery, a rear housing closure attached to tile other end of said wall having a protrusion, a plate having an opening connected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate having a concave depression with its surface substantially parallel to the surface of said first depression, a transducer element mounted in said protrusion and connected to said diaphragm, said front closure having a plurality of passages extending therethrough, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency. 7 a

8, A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a first depression in said front housing closure, said front housing closure becoming progressively thicker from the edge of said cone-shaped depression to its periphery, a rear housing closure attached to the other end of said wall, a plate having an opening connected to said wall so as to divide the housing into two chambers, 21 diaphragm mounted in said opening of said plate having a second depression with its surface substantially parallel to the surface of said first cone-shaped depression, a transducer element mounted in said rear closure and connected to said diaphragm, said front closure having a first plurality of passages, said front closure having a second plurality of passages located substantially diametrically opposite said first plurality of passages, each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal tonne-half of the wave length of a particular chosen frequency.

9. A microphone comprising a housing wall, a front housing closure attached to one end of said wall becoming progressively thicker from the central portion to its periphery, a rear housing closure attached to the other end of said wall, means arranged to divide the housing into two chambers including a plate having an opening therein and a diaphragm mounted in said opening of said plate having its surface substantially parallel to the'inner surface of said first closure, a transducer element mounted in said rear housing closure and connected to said diaphragm, said front closure having a first plurality of passages, said front closure having a second plurality of passages, each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency.

ii). A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a first cone-shaped depression in said front housing closure,

a rear housing closure attached to the other end of said wall having a protrusion, a plate having an opening connected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate having a second cone-shaped depression with its surface substantially parallel to the surface of said first cone-shaped depression, a transducer element mounted in said protrusion and connected to said diaphragm, said front closure having a first plurality of passages adjacent its periphery, said front closure having a secondplurality of passages adjacent its periphery and located substantially diametrically opposite said first plurality of passages; .each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a first plurality of passages in substantial alignment with said first plurality of passages in said front closure, 21 second plurality of passages substantially diametrically opposite said first plurality of passages in substantial alignment "with said second plurality of passages in said front closure, the relationship as to effective spacing and impedance between passages in said rear closure being the same as that between corresponding substantially aligned passages in said front closure, a first phase shifting element covering said first plurality of passages in said rear closure, and a second phase shifting element covering said second plurality of passages in said rear closure.

- 11. A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a first depression in said front housing closure, a rear housing closure attached to the other end of said wall, a plate having an opening connected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate having a second depression with its surface substantially parallel to the surface of said first depression, a transducer element mounted in said rear housing closure and connected to said diaphragm, said front closure having a. first plurality of passages, said front closure having a second plurality of'passages, each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an efiective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a first plurality of passages equal in number and spacing apart as said first plurality of passages in said front closure, a second plurality of passages equal in number and spacing apart as said second plurality of passages in said front closure, the relationship as to effective spacing and impedance between passages in said rear closure being the same as that between corresponding passages in said front closure, a first phase shifting element covering said first plurality of passages in said rear closure, and a second phase shifting element covering said second plurality of passages in said rear closure.

12. A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a first depression in said front housing closure, a rear housing closure attached to the other end of said wall, a plate having an opening connected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate having a second depression with its surface substantially parallel to the surface of said first depression, a transmitter connected to said diaphragm, a piezoelectric element mounted in said rear housing closure and connected to said transmitter, said front closure having a first plurality of passages, said front closure having a second plurality of passages,'each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a first plurality of passages equal in number and spacing apart as said first plurality of passages in said front closure, a second plurality of passages equal in number and spacing apart as said second plurality of passages in said front closure, the relationship as to effective spacing and impedance between passages in said rear closure being the same as that between corresponding passages in said front closure.

13. A microphone comprising a housing wall, a front housing closure attached to one end of said wall having an inner convex surface, a rear housing closure attached to the other end of said wall, a plate dividing the housing into two chambers having an opening therein confronting the convex surface of the first closure, a diaphragm mounted in said opening of said plate with its surface substantially parallel to the convex surface of said first closure, a transducer element mounted in said rear housing closure and connected to said diaphragm, said front closure having a plurality of passages located on one side of the convex surface, and a second plurality of passages located on the other side of the convex surface, the first plurality of passages being associated with the second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a plurality of passages associated with said plurality of passages in said front closure, the relationship as to effective spacing and impedance between passages in said rear closure being the same as that between corresponding associated passages in said front closure, and a phase shifting element covering said plurality of passages in said rear closure.

14. A microphone comprising a housing wall, a front housing closure attached to one end of said wall having an inner convex surface, a rear housing closure attached to the other end of said wall, a. plate dividing the housing into two chambers having an opening confronting the convex surface, a diaphragm mounted in said opening of said plate with its surface substantially parallel to the convex surface of said first closure, a transducer element mounted in said rear housing closure and connected to said diaphragm, said front closure having a first plurality of passages located on one side of the convex surface, said front closure having a second plurality of passages located on the other side of the convex surface, each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a first plurality of passages associated with said first plurality of passages in said front closure, a second plurality of passages associated with said second plurality of passages in said front closure, the relationship as to effective spacing and impedance between passages in said rear closure being the same as that between corresponding associated passages in said front closure.

15. A microphone comprising a housing wall, a front housing closure attached to one end of said wall, a rear housing closure attached to the other end of said wall having a protrusion, a plate having an opening connected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate, the inner surface of said front closure adjacent said diaphragm being substantially parallel to the one surface of said diaphragm, a transducer element mounted in said protrusion and connected to said diaphragm, said front closure having a plurality of passages extending therethrough, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency. I

16. A pressure gradient microphone comprising means defining a first sound chamber having a plurality of openings in a first region and a plurality of openings in a second region spaced from the first region on a first common plane by an effective distance at least equal to a half Wave length at the highest frequency within the response range of the microphone, means defining a second sound chamber having a plurality of orifices in a first region and a plurality of orifices in a second region spaced from the first region on a second common plane, the orifices of the first region confronting the openings of the first region and the orifices of the second region confronting the openings of the second region, and electro-mechanical transducing means including a diaphragm mounted between the first and second chambers, said diaphragm confronting the portion of the first sound chamber defining means disposed between the first and second regions thereof and the portion of the second sound chamber defining means disposed between the first and second regions thereof.

17. A pressure gradient microphone comprising the elements of claim 16 in combination with sound phase shifting means disposed in series with the orifices in the first and second region of the second sound chamber defining means.

18. A pressure gradient microphone comprising the elements of claim 17 wherein the openings in the first and second regions of the first sound chamber defining means have larger cross sections adjacent to the diaphragm and smaller cross sections remote therefrom.

19. A pressure gradient microphone comprising the elements of claim 17 wherein the cross section of the openings in the first and second region of the first sound chamber are approximately of the same size and the thickness of the first sound chamber defining means is greater at openings remote from the diaphragm than at openings adjacent to the diaphragm.

20. A pressure gradient microphone comprising the elements of claim 18 wherein the cross sections of the orifices in the first and second regions of the second sound chamber defining means are greater adjacent to the diaphragm than remote from the diaphragm.

21. A pressure gradient microphone comprising the elements of claim 19 wherein the orifices of the first and second regions of the second sound chamber are approximately of the same cross section and the thickness of the second sound chamber is greater at those orifices remote from the diaphragm than at those orifices adjacent to the diaphragm.

22. A microphone comprising a housing wall, a front housing closure attached to one end of said wall having a first depression in said front housing closure, said front housing closure becoming progressively thicker from the edge of said depression to its periphery, a rear housing closure attached to the other end of said wall having a cylindrical protrusion, said rear housing closure becoming progressively thicker from the edge of the cylindrical protrusion to its periphery, a plate having an opening connected to said wall so as to divide the housing into two chambers, a diaphragm mounted in said opening of said plate having a second depression of substantially similar shape as and with its surface substantially parallel to the surface of said first depression, a transducer element mounted in said protrusion and connected to said diaphragm, said front closure having a first plurality of passages, said front closure having a second plurality of passages located substantially diametrically opposite said first plurality of passages, each passage of said first plurality of passages being associated with a passage of said second plurality of passages, each of said associated passages being of such diameter, length and distance apart as to have an effective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a first plurality of passages in substantial alignment with said first plurality of passages in said front closure, 2. second plurality of passages substantially diametrically opposite said first plurality of passages in substantial alignment with said second plurality of passages in said front closure, the relationship as to effective spacing and impedance between passages in said rear closure being 11 the same as that between corresponding substantially aligned passages in said front closure, a first phase shifting element covering said first plurality of passages in said rear closure, and a second phase shifting element,

covering said second plurality of passages in said rear closure. p

23. A microphone comprising a housing wall, a front 7 housing closure attached to one end of said wall having a first depression in said front housing closure, said front housing closure becoming progressively thicker from the edge of said depression to its periphery, a rear housing closure attached to the other end of said wall having a protrusion, said rear housing closure becoming progressively thicker from the edge of the protrusion to its periphery, a plate having an opening connected to said wall so as to divide the housing into two chambers, a

' diaphragm mounted in said opening of said plate having a surface substantially parallel to the surface of said first depression, a piezoelectric element mounted in said protrusion and connected to said diaphragm, said front closure having a first plurality of passages, said front closure having a second plurality of passages located substantially diametrically opposite said first plurality sages being associated with a passage of said second plurality of passages, each of said associated passages being' of such diameter, length and distance apart as to have an elfective spacing equal to one-half of the wave length of a particular chosen frequency, said rear closure having a first plurality of passages equal in number and spacing from the periphery as said first plurality of passages in said front closure, a second plurality of passages substantially diametrically opposite said first plurality of passages equal in number and spacing from the periphery asisaid second plurality of passages in said front closure, the relationship as to efiective spacing and impedance between passages in said rear closure being the same as that between corresponding passages in said front closure, and phase shifting elements covering said plurality of passages in said rear closure.

References Cited in the file of this patent UNITED STATES PATENTS 2,035,108 Weil Mar. 24, 1936 2,463,762 Gianni Mar. 8, 1949 2,587,684 Bauer Mar. 4, 1952 UNITED STATES PATENT armor:

CERTIFICATE 6F CORRECTIQN Patent No. 2,921,993 January 19 1960 Wayne A. Beaverson It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 32, for "pair of sound openings of groups or entrances" read pair of sound openings or groups of entrances Signed and sealed this 21st day of June 1960 (SEAL) Attest:

KARL H. AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents 

